Metal substrate with a corrosion-resistant coating produced by means of plasma polymerisation

ABSTRACT

The invention relates to a metal substrate with a corrosion-resistant coating produced by means of plasma polymerisation, wherein the substrate is subjected to mechanical, chemical and/or electrochemical smoothing in a pre-treatment step and is subsequently exposed to a plasma at a temperature of less than 200° C. and at a pressure of 10 −5  bis 100 mbars, whereby in a first step the surface is activated in a reducing plasma and in a second step the polymer is separated from a plasma containing at least one hydrocarbon or silico-organic compound which can be vaporized in plasma conditions, optionally contains oxygen, nitrogen or sulphur and can contain fluorine.

SPECIFICATION

[0001] The invention concerns a process for the corrosion-resistantcoating of metal substrates by means of plasma polymerization. Theprocess is especially suitable for the corrosion-resistant coating ofaluminum and aluminum alloys.

[0002] Ever since research began addressing the production ofplasma-applied polymer layers by means of polymerization processes whichproduce the energy needed for polymerization by adding gaseous monomersto a gas discharge process, there has been no lack of attempts todeposit these layers in such a manner that they are able to protect thecoated surface from different types of attack. This function is by nomeans trivial, since plasma-applied polymer layers are decidedly thinlayers, measuring in the nanometer range up to a few micrometers. Notonly were scratch-resistant layers developed, e.g. for opticalfunctional elements made of plastic (WO-A-8504601), but there were alsoattempts, with moderate success, to use this type of layer to protectmetallic materials. Even types of attack that must be considered lessthan severely corrosive were withstood by these layers for only veryshort periods of time.

[0003] In all the experimentation that is known at this time involvingaluminum materials, oxide layers are used to promote adhesion inoxidizing plasma arrangements, and this is analogous to conventionallacquering processes, however, it is also analogous to the preparationof surfaces before gluing, where an oxide layer, which has usually beenproduced by anodic oxidation, is used. Activation of the boundary layer,which is desirable for good adhesion, is achieved, if at all, byintercalating substances of a foreign nature. Bonding is frequentlycarried out solely by means of adhesive forces. Experience has shownthat such coating or gluing systems exhibit only moderate imperviousnessto infiltration, because water vapor arising by diffusion or permeationprocesses weakens the bond between the material and the coating.

[0004] Plasma polymerization, on the other hand, is a process that iscapable of coating solid objects by the action of a plasma on an organicmolecule in the gas phase, whereby the coatings created in this mannerare primarily organic in character and have excellent properties. Plasmapolymerization belongs to the category of low-pressure plasma processes,and is used increasingly for industrial purposes. The great interest inthis technology derives from the advantages of a rapid, contact-free,dry chemical coating process, which furthermore puts little stress onthe work piece.

[0005] Plasma-applied polymer layers deposited by low-temperatureplasmas, hereinafter referred to as plasma polymers, are distinguishedby the following characteristics:

[0006] Plasma polymers are often three-dimensionally highly cross-linkedand insoluble and swell only slightly or not at all, and are potentiallygood barriers to diffusion.

[0007] Compared to conventionally manufactured polymers, their highdegree of cross-linking makes them unusually stable thermally,mechanically, and chemically.

[0008] The layers adhere well to most substrate materials, and have ahigh density and are free of micropores.

[0009] The layers are usually amorphous in structure, with a smoothsurface that conforms to the shape of the substrate.

[0010] The layers are very thin, and the thickness of the layer amountsto only a few 100 nm down to 10 nm.

[0011] The process temperatures are low, i.e. room temperature up toapproximately 100° C., especially up to approximately 60° C.

[0012] On the other hand, no processes are yet known with which metalsubstrates, especially substrates comprising aluminum materials, can bemade corrosion-resistant by coating with a plasma polymer.

[0013] Ribbed pipes made from the material AlMgSi0.5 are frequently usedin condensing boilers. When used under extreme conditions and in areasapproaching the limits for allowable gas composition, such ribbed pipesdo not always exhibit sufficient corrosion resistance.

[0014] The formation of corrosion products results in malfunctions onthe gas side in the vicinity of the pipe ribs and, in advanced stages, areduction of the heat exchanger surface on the combustion gas side, aswell.

[0015] Conventional means for protecting against corrosion, which arestate of the art, cannot be adopted for several reasons. Processes suchas phosphatizing or chromizing bring about the continuous emission ofheavy metal ions into the environment and must be excluded due to thelikelihood of more restrictive anticipated legislation on waste waterdisposal.

[0016] Lacquer systems do not constitute a viable alternative, either.Lacquers used as a means for protecting surfaces compromise thermalconductivity, which in the present case can be tolerated only withinvery narrowly defined limits. Furthermore, in conventional lacquercoatings, the diffusion of water vapor leads to infiltration of theprotective layer. Subsequent condensation on the metal surface causesthe layer to separate in such systems, thereby accelerating the processof corrosion, as is known for localized types of corrosion.

[0017] Coating such ribbed pipes for heat exchangers with a plasmapolymer would, in and of itself, be desirable. However, in experimentaltrials, in this connection, corrosion-resistant coatings were notachieved. As a rule, the plasma polymers were found not to adhere firmlyenough to the metal surface, and infiltration of the coating was foundto occur more or less rapidly, with the result that it soon showed signsof separating.

[0018] A process for the surface coating of silver objects is known from[the German patent application] DE-A-42 16 999, in which the surface isfirst treated with a stripping plasma, and the surface is then coatedwith a plasma polymer, whereby an initial coupling layer, a surfacelayer to prevent permeation on top of that, and finally a sealant layerare produced. Ethylene and vinyltrimethylsilane are especially used forthe coupling layer, ethylene for the layer preventing permeation, and,for the sealant, hexamethyldisiloxane in conjunction with oxygen as aplasma forming monomer, whereby a continuous transition occurs betweenthe plasma forming monomers. The coatings are largely scratch resistant,and they provide good protection against tarnish; in certainformulations, however, they can be susceptible to removal by cleansers.A coating on aluminum substrates fails to provide corrosion-resistantlayers.

[0019] On the whole, it would be desirable to have a process availablewith which a long-lasting and corrosion-resistant plasma polymer coatingcan be applied to metal materials, especially aluminum materials.

[0020] This goal is achieved by a process of the type mentioned at theoutset, where the substrate undergoes a pre-treatment step of smoothingby mechanical, chemical, and/or electrochemical means, after which, at atemperature of less than 200° C. and a pressure of 10⁻⁵ to 100 mbar, itis exposed to a plasma, whereby, in an initial step, the surface isactivated in a reducing plasma and, in a second step, the plasma polymeris deposited from a plasma that optionally contains at least onehydrocarbon or organosilicon compound, which can be vaporized under theconditions of the plasma, containing oxygen, nitrogen, or sulfur, andwhich may contain fluorine atoms.

[0021] A surprising finding was that the problem of insufficientadhesion of the coating to the metal surface is solved by thecombination of a smoothing pre-treatment of the metal substrate which isto be coated with a plasma treatment. The plasma treatment, in turn,consists of 2 steps, namely, first treatment of the surface by areducing plasma which acts as a surface stripper, and a second step, inwhich the actual coating is applied directly to the metal layer that hasbeen pre-treated by the plasma.

[0022] The pre-treatment, especially the smoothing of the surface of themetal substrate, can be carried out by mechanical, chemical, orelectrochemical means. Especially preferred are combinations comprisingboth mechanical and chemical smoothing. In any event, electrochemicalsmoothing can be undertaken after mechanical and/or chemical smoothing,if the particular metal substrate in question will permit this. In thecase of ribbed pipes, for example, the electropolishing method is not asuitable surface treatment for physical/technical reasons. Here one hasto rely on chemical methods, such as acid or alkaline pickling.According to [the German patent document] DE-C-40 39 479, it is alsopossible to use a combination of pickling and mechanical disturbance ofthe surface by wiping, brushing, abrasive blasting processes, etc.,whereby the work piece, in particular, is impacted by a jet of liquidcontaining both the pickling solution and particles with an abrasiveaction.

[0023] The pickling methods used in smoothing the surface are chemicalprocesses in which aggressive chemicals are used to remove, primarily,layers of oxide, rust, and scale from the particular metal surface inquestion. Pickling solutions are usually acids that attack both thecovering layers and the metal itself. Pickling is not a single process.Instead, different chemical and physical processes occur simultaneouslyand in succession. These processes are often electrochemical in nature,and involve the formation of local elements [sic] between the metaloxides and the metal surface.

[0024] Electropolishing is a method used for obtaining shiny metalsurfaces, and it electrolytically removes raised bumps and ridges.

[0025] Chemical bright pickling is highly developed as a process forsmoothing off surface roughness, especially for aluminum. It isfundamentally more important than electropolishing. There are manychemical bright picklers for aluminum.

[0026] Most chemical solutions for producing gloss are formulated on thebasis of phosphoric acid. The addition of nitric acid producesreflecting surfaces as well as improving the surface quality. Theaddition of sulfuric acid makes the metal dissolve more quickly andresults in the improved evening out of irregularities. There are otheradditives that can further speed up the rate at which the metal isstripped and extend the length of time the pickling bath will remainoperational.

[0027] The effects of pickling, including bright pickling, can be madeless variable and faster when they are used in conjunction withmechanical methods of surface treatment. According to the invention,such a combination of mechanical and chemical methods of surfacetreatment, as those described in [the German patent document] DE-C-40 39479, is especially utilized for smoothing.

[0028] Due to the amphoteric properties of aluminum and its alloys, itis also possible to use alkaline solutions to clean and pickle them.

[0029] Generally speaking, the treatment to smooth the surface resultsin an average mean roughness of less than 350 nm, preferably less than250 nm. By using electropolishing methods, and especiallyelectropolishing after a mechanical/chemical smoothing process, anaverage mean roughness of less than 100 nm can be obtained.

[0030] However, surfaces that have been smoothed by such methods arestill not optimally suited for the application of a plasma polymer. If aplasma polymer is applied after mechanical/chemical and/orelectrochemical smoothing, it will not yet hold up for the desiredlength of time under corrosive conditions. In order to achieve thatgoal, one more surface treatment is needed, using a reducing plasma,especially a hydrogen plasma. Said plasma treatment takes place attemperatures of ≦200° C. and pressures of ≦100 mbar, especially ≦100° C.and ≦10 mbar. Other gases can be added to the hydrogen as the plasmacarrier, e.g. hydrocarbons, and olefins in particular, as describedbelow, as well as oxygen, nitrogen, or argon, whereby the reducingcharacter must always be maintained.

[0031] This plasma treatment produces an activated surface. The reducingconditions presumably cause a decrease in the aluminum oxide layerand/or aluminum hydroxides which are near the surface on the metalsurface, so that points of attachment arise where reactive bonding of aplasma polymer, subsequently applied, can arise directly on the metal. Afurther side effect is that the surface undergoes additional smoothingby the plasma treatment.

[0032] A plasma polymer is precipitated onto the plasma-treated surface,preferably under further reducing conditions at first. The maincomponent of this plasma polymer is a hydrocarbon and/or anorganosilicon compound, which can contain oxygen, nitrogen, or sulfuratoms, whereby said hydrocarbon or organosilicon compound has a boilingpoint such that it can be vaporized under the temperature and pressureconditions prevailing in the plasma coating chamber. Substances thatwould qualify for this purpose are primarily alkanes, alkenes, aromatichydrocarbons, silanes, siloxanes, silazanes, and silathianes, preferablysiloxanes. The utilization of hexamethyldisiloxane andhexamethylcyclotrisiloxane is especially preferred. Other compounds arehexamethyldisilazane and hexamethylcyclotrisilazane, as well ashexamethyldisilathiane. It is also possible to use higher homologs ofthese compounds and mixtures of such compounds, or their partially orcompletely fluorinated derivatives.

[0033] Hydrocarbons, especially olefins, e.g. ethylene, propene, andcyclohexene, can serve as co-monomers for the formation of plasmapolymers from organosilicon monomers. Silanes, especially organosiliconcompounds containing vinyl groups, can also be used as co-monomers, e.g.vinyltrimethylsilazane. These unsaturated monomers can be added in fixedor variable proportions to the organosilicon compound containing O, N,or S atoms, with one possibility being to add them in gradualincrements. For example, when forming the plasma polymer step by step,it is possible first to form a transition layer on the metal surface,consisting solely or predominantly of the organosilicon compound, andsubsequently to add the hydrocarbon. Another possibility is to proceedin the reverse order. Thus, the properties of the plasma polymer coatingcan be modified so as to provide optimal adhesion to the metal substrateand/or optimal resistance to corrosive substances. Such a manner offorming the layer in stages is known, for example, from [the Germanpatent application] DE-A-42 16 999.

[0034] During plasma polymerization, other gases in addition to thesemonomers can also be added, e.g. oxygen, nitrogen, or argon, in order tomodify the properties of the plasma and the plasma polymer.

[0035] Generally speaking, plasma polymerization takes place at atemperature of ≦200° C., preferably ≦100° C., and especially about 60°C. The pressure in the plasma coating chamber is usually ≦10 mbar.

[0036] The layer formed on the metal substrate by plasma polymerizationexpediently has a thickness of 100 nm to 10 μm. But it is not a problemto create layers with a thickness of less than 100 nm for specialpurposes.

[0037] Unlike other coatings, including plasma polymer coatings that areapplied by using other methods, the surface is smoothed according to theinvention by a pickling process that evens it out, and whoseeffectiveness is enhanced and rendered less variable by a superimposedlight mechanical component. As a result, mechanical cramping of thepolymer coating on the metal substrate caused by the relatively highroughness of the substrate occurs less often, and instead there isbonding of a rather more chemical nature to free valences of the exposedmetal surface that has been stripped by caustic agents. Generallyspeaking, a bright, almost mirror-like finish that appeals to the eye isobtained on non-structured metal surfaces. The result, in particular, isthat, with its thickness being what it is, the coating is no longer“swallowed up” by the surface structures of a rough metal surface, andinstead a uniform, smooth layer is formed.

[0038] Corrosion resistance several times better than that obtained forindustrial surfaces is achieved according to the invention.

[0039] A further increase in long-term corrosion resistance is achievedby incorporating a corrosion inhibitor which can be vaporized in avacuum, preferably into the bottom-most layer of the plasma polymercoating. In a departure from previously available findings, it is notessential for such a corrosion inhibitor to be applied directly to thesurface of the substrate, i.e. to be located directly at the level whereadhesion takes place, thereby weakening it. Instead, it is effective ata distance, especially in connection with the use of conductivepolymers. Suitable polymers of this type are, for example, polyanilineswhich have low vapor pressure [sic] in a vacuum, or which can beintroduced into the plasma polymer in the most finely distributed form,in quantities of 0.1 to 1% by weight.

[0040] In addition to being used with aluminum materials, the technologydescribed above can be applied to other metal materials as well,especially those with a tendency to form an oxide layer on theirsurface.

[0041] The process according to the invention can also be used forapplying a plasma polymer primer to a metal substrate to whichadditional coatings are subsequently applied. This allowscorrosion-resistant coatings of considerable thickness to be obtainedfor the most diverse purposes, and which have sufficient thickness tostand up to abrasive wear. Ormocers are especially well suited for thispurpose. The structural composition of ormocer coatings is similar tohighly cross-linked plasma polymer coatings, but they can be formedwithout the relatively slow vacuum coating process. Here, the typicalthickness of the layer is on the order of 1 to 100 nm. Using them incombination, corrosion properties are obtained of a quality that iscomparable to and as good as with plasma polymer coatings alone.

[0042] The process according to the invention is especially well suitedfor coating aluminum materials, whereby the resulting corrosionresistance renders the aluminum material especially suitable for beingemployed as a heat exchanger and for manufacturing ribbed pipes for heatexchangers in condensing boilers.

EXAMPLE

[0043] Rectangular samples of the material AlMgSi0.5 were used as thetest material. The samples initially underwent a multi-stage cleaningprocess to remove foreign substances such as oils and greases. Thesurface of the metal pieces was then treated using a combined picklingand electro-polishing process.

[0044] The samples were first cleaned mechanically by a brush in a soapsolution with a neutral pH, then they were rinsed off, and again treatedwith the soap solution for 30 minutes at t=70° C. in an ultrasonic bath.After more rinsing under running water and drying with hot air, theywere degreased in the ultrasonic bath with acetone till they were asclean as possible and then they were dried with hot air.

[0045] The metal samples were next pickled in a pickling solution of46.0 parts of water, 50.0 parts of concentrated nitric acid, and 4.0parts of hydrofluoric acid for 120 s at room temperature. After rinsingwith water and ethanol, the test piece was then polishedelectrochemically. A mixture of 78 ml of 70% to 72% chloric acid, 120 mlof distilled water, 700 ml of ethanol, and 100 ml of butyl glycol servedas the electrolyte. Electropolishing was carried out for a period of 180s at an electrolyte temperature of −15 to +8° C., with a polishingcurrent of 5 to 18 A/dm² and a polishing voltage of 19 to 11 V.

[0046] Immediately following electropolishing, the sample was rinsed offwith water and treated for 10 minutes with cold water in an ultrasonicbath. Finally, it was dried with hot air.

[0047] Prior to surface smoothing, the test piece had a matte surfacewith a mean roughness of 0.570 μm (averaged over 5 measurements). Afterthe electropolishing, the mean roughness was less than 100 nm. Thesurface had a mirror-bright finish.

[0048] The plasma treatment was carried out in conventional plasmapolymerization equipment, where the monomer gas was introduced into thelow-pressure container and excited by a high-frequency alternatingcurrent and/or microwave energy to form a plasma.

[0049] In an initial plasma treatment step, the aluminum work piece wassubjected to a hydrogen plasma at 60° C. and 50 mbar for 120 s. Thehydrogen was successively replaced by feeding in hexamethyldisiloxane ata pressure of 10 mbar. The volume flow rate was 500 ml/min, and themaximum power output was 5 KW. The layer was applied at a thickness of500 nm.

[0050] The example was varied by first having a plasma polymersubstance, comprising ethylene as the monomer, applied to the metalsurface during the plasma polymerization, to which increasing quantitiesof hexamethyldisiloxane were then added, until the ethylene wascompletely replaced.

[0051] In other trials, the monomers were supplemented by the additionof oxygen and nitrogen.

[0052] In all of these processes, highly corrosion-resistant, thin,transparent layers were deposited on the surface of the sheet aluminum,which retained its mirror-bright finish.

[0053] Using electromicroscopy, it was found that the plasma polymerlayer had bonded well to the metal surface. The plasma polymer layer isamorphous and practically flawless, i.e. it has no pores or inclusions.

[0054] The corrosion properties of the sheet aluminum, which had beencoated in this manner, were tested in 25% sulfuric acid at roomtemperature and 60 to 70° C., and then again in 20% nitric acid at roomtemperature. In the tests, which were conducted over several hours, allthe samples were found to be resistant. No migration of the test liquidinto the coating occurred, much less infiltration of the coating by theliquid. No separation phenomena were observed.

[0055] The pieces of sheet aluminum coated according to the inventionwere found to be absolutely corrosion-proof at 350° C. under conditionssuch as those that prevail in a heat exchanger for condensing boilers.Furthermore, they have reduced surface tension, and therefore tend togenerate fewer mineral deposits, e.g. in the form of boiler scale. Thereduced surface tension also provides protection against biologicalinfestation, for example in work pieces that are exposed to seawater.

1. A metal substrate with a corrosion-resistant coating produced bymeans of plasma polymerization, comprising the steps of: smoothing thesubstrate using mechanical, chemical, and/or electrochemical smoothing;producing an activated surface of the substrate by subjecting thesubstrate to a reducing plasma at a temperature of less than 200° C. anda pressure of 10⁻⁵ to 100 mbar; and depositing a plasma polymer on thesubstrate from a plasma that contains at least one hydrocarbon ororganosilicon compound, optionally containing oxygen, nitrogen, orsulfur, that can be vaporized under the plasma conditions, and which maycontain fluorine atoms.
 2. A metal substrate according to claim 1,wherein the metal substrate is aluminum or an aluminum alloy.
 3. A metalsubstrate according to claim 1, wherein the smoothing step comprisessubjecting the metal substrate to a combination of a mechanical surfacetreatment and pickling.
 4. A metal substrate according to claim 1,wherein the smoothing step comprises electrochemically polishing themetal substrate.
 5. A metal substrate according to claim 1, wherein inthe smoothing step, obtaining an average mean roughness of the metalsubstrate of less than 350 nm.
 6. A metal substrate according to claim1, wherein in the step of producing an activated surface, thetemperature is less than 100° C.
 7. A metal substrate according to claim1, wherein in the step of activating the surface, the surface isactivated by a hydrogen plasma at a pressure of ≦100 mbar.
 8. A metalsubstrate according to claim 1, wherein in the plasma polymer depositingstep, the organosilicon compound includes a siloxane, silazane, orsilathiane.
 9. A metal substrate according to claim 1, wherein in theplasma polymer depositing step, the organosilicon compound comprises asiloxane, or a siloxane comprising hexamethyldisiloxane orhexamethylcyclotrisiloxane.
 10. A metal substrate according to claim 1,wherein in the step of producing an activated surface, the plasmaincludes a hydrocarbon, or a hydrocarbon comprising an olefin.
 11. Ametal substrate according to claim 10, wherein the hydrocarbon comprisesethylene, propylene, or cyclohexene.
 12. A metal substrate according toclaim 1, wherein in the plasma polymer depositing step, the depositiontakes place at a pressure of ≦10 mbar under initially reducingconditions.
 13. A metal substrate according to claim 1, wherein in thestep of producing the activated surface, feeding oxygen, nitrogen,and/or a noble gas into the plasma.
 14. A metal substrate according toclaim 1, wherein in the plasma polymer depositing step, the plasmapolymer layer is applied at a thickness of 100 nm to 1 μm.
 15. A metalsubstrate according to claim 1, further comprising introducing acorrosion inhibitor into the plasma polymer.
 16. A metal substrateaccording to claim 15, wherein the corrosion inhibitor comprises apolyaniline in a quantity of 0.1 to 1% by weight.
 17. A metal substrateaccording to claim 1, further comprising applying an additional coatingto the plasma-coated metal substrate.
 18. A metal substrate according toclaim 1, wherein the substrate comprises an aluminum heat exchanger. 19.A metal substrate of claim 18, wherein the aluminum heat exchangercomprises ribbed pipes.